The delivery or injection of fluidic materials to and removal of fluidic materials from a selected site may be performed in a number of different medical procedures. In the field of ophthalmology, for example, intraocular injections may be administered for many reasons. Some of these reasons include: (1) the injection of antibodies to treat endothalmitis or prevent its onset; (2) the injection of Transforming Growth Factor Beta (TGFB) or other growth factors to treat macular disorders; (3) the injection of Tissue Plasminogen Activator (tPA) into the subretinal space to dissolve blood clots; (4) the injection of liquids and gases into the subretinal space to facilitate subretinal surgery; (5) the injection of viscoelastic substances to dissect preretinal membranes; and (6) the injection of gases into the vitreous cavity for pneumatic retinal pexy.
When injections are administered to delicate tissue, e.g., intraocular injections, the surgeon must control the following: injection rate, total volume administered, and location of the injected substance. Similar concerns exist for aspiration of fluids from delicate tissues. The case of viscodissection is described below to illustrate these requirements.
Viscodissection is a technique where preretinal membranes are hydraulically separated from the retina using a viscoelastic substance. This substance, typically sodium hyaluranate, is delivered between the preretinal membrane and the retina using a syringe and a small gauge bent needle. The fluid creates a working space underneath the retina. Many surgeons find it difficult to hold the needle tip steady while injecting the fluid and inadvertent motion of the needle can cause damage to the retina or other surrounding tissues. Further, injecting too much fluid between the preretinal membrane and the retina, or injecting the fluid too fast, can also cause retinal damage which could lead to retinal detachment. Similarly, aspiration of unwanted fluid from these delicate tissues requires steady and measured suction.
There are some devices which facilitate the delivery of fluidic materials to delicate tissue. For example, U.S. Pat. No. 5,370,630 discloses a device that uses pneumatic energy to cause the injection of fluidic material into body tissue. The plunger of this device is driven by pneumatic pressure instead of finger pressure, thereby allowing the surgeon to better control the injection rate, volume, and location. A number of other syringe adapters and pneumatic pressure sources are currently available. Such devices typically have a piston displacement v. time curve as shown in FIG. 13.
While more effective than manual instruments, existing pneumatic fluid delivery devices often cannot meet the requirements of the surgeons for precision because they cannot control the "jetting" of material emitted from the needle, require a relatively high amount of pneumatic pressure to operate, and cannot be precisely controlled for very low doses. "Jetting," or turbulent flow of the fluidic material, occurs when the fluid emitted through the hole in the needle is forced out under relatively high pressure by a rapidly accelerating plunger motion. Jetting is undesirable because it may damage the tissue to which the fluidic material is being delivered. Similarly, existing aspiration devices do not meet the needs of surgeons for aspiration of delicate tissue because they do not offer adequate control of the suction force in strength, location, and volume.
The major deficiencies of existing devices are caused by internal friction. As with any dynamic system, friction is present in devices designed to deliver fluids. With air cylinders, as in existing injectors, friction due to o-rings rubbing against the walls of the cylinder can be very difficult to control. All o-ring type piston-cylinder assemblies have an inherent problem with initial static friction created by at least two sources. One is static friction due to material properties; the other is commonly referred to as "stiction." Stiction is the frictional force due to a compression of the o-ring incurred when the piston-cylinder assembly has been sitting unused for some time. The ideal control for injection is a constant velocity, linear displacement travel of the piston. In prior-art devices, the stiction and static friction in the air cylinder result in uncontrollable motion, which is illustrated in FIG. 13. As pressure is increased to initiate motion of the piston, initially nothing happens. Then, there is an almost instantaneous movement of the piston (from zero to a level indicated by reference numeral A) as the friction and stiction forces are overcome. This jump in motion results in the jetting of the fluid being delivered. This initial jump can be as much as 75% of the total stroke (reference numeral B) of the piston. Thus, no matter how well the delivered air supply is controlled, the result is a quick, pulse-like delivery of fluid and potential damage to the tissue into which the fluidic material is being injected.
Existing aspiration devices typically do not offer precise control of the rate and volume of fluid being aspirated and thus are not very useful around sensitive tissues. Specifically, current aspiration devices used in intraocular surgery, such as those used in vitrectomy procedures, use vacuum control. In such devices, the vacuum level is controlled, not the rate or volume of material aspirated. Aspiration devices typically used in cataract surgery suffer from several limitations. First, such devices cannot accurately remove fluids in the sub-microliter range and thus cannot be used around delicate tissue such as the retina. Second, such devices are flow-controlled rather than volume-controlled, that is, the surgeon can control the suction rate at which material is removed, but not the volume. Thus, existing devices cannot be used to remove a precise volume of material as may be required in surgeries such as the treatment of sub-retinal hemorrhages.
Accordingly, a need has arisen for a device capable of delivering and removing fluidic materials from delicate body tissue. Further, it would be desirable if the device could deliver a user-settable volume of fluid; deliver and remove fluids at a volume rate precisely controlled by the operator; allow the use of a relatively low pressure pneumatic source; reduce jetting of fluid emanating from the needle; and further minimize the risk of tissue damage that results from manually operated syringes, existing pneumatic syringes, and existing aspiration devices.